1. Field of the Invention
The present invention relates to a lithium ion secondary battery, and more particularly to a lithium ion secondary battery having a safety vent responsive to temperature and pressure for improved safety.
2. Description of the Prior Art
As generally known in the art, a lithium ion secondary battery includes an electrode assembly having a positive electrode plate with positive electrode active materials attached thereto, a negative electrode plate with negative electrode active materials attached thereto, and a separator positioned between the positive and negative electrode plates to prevent a short circuit and to allow movement of lithium ions. The positive and negative electrode plates and the separator may be wound into a jelly roll configuration. The secondary battery may also include an electrolyte for enabling lithium ions to move; a can which contains the electrode assembly and the electrolyte and which is then sealed; and a cap assembly for covering the can and preventing the electrode assembly from escaping.
Such a lithium ion secondary battery may be manufactured as follows: a positive electrode plate having positive electrode active materials attached thereto, a negative electrode plate having negative electrode active materials attached thereto, and a separator are laminated and wound into a jelly roll configuration and placed into a square type can. Then, a cap assembly is welded to the top of the can to seal it and an electrolyte is injected into the can. A bare cell then may be charged and inspected and various safety devices may be attached to the bare cell to complete a conventional battery pack.
A constant voltage/current charging method is used for lithium ion secondary batteries and overcharging does not occur as long as the charging voltage is correctly controlled in chargers. However, abnormal charging sometimes occurs as the chargers are damaged or erroneously operated. When this happens, the electrical potential of positive electrode active materials, e.g., lithium cobalt oxide (LiCoO2), continuously rises causing unceasing rise of the battery voltage and an abnormal heating phenomenon.
Safety measures against such overcharging include a positive temperature coefficient (PTC) thermistor, a separator having a shutdown function, and a safety vent actuated by gas generation. As used herein, a safety vent of a square-type lithium ion secondary battery generally refers to a relatively thin region formed on the bottom surface of the can or on the cap assembly which is adapted to fracture during severe swelling caused by gas generation and allows gas to be discharged to outside the battery.
The gas generation occurs when the amount of lithium carbonate (Li2CO3) added to form positive electrode active materials, such as LiCoO2, exceeds the stoichiometry. Particularly, the extra lithium carbonate remains in a non-reacted state within the positive electrode active materials (lithium cobalt oxide) and decomposes to produce carbonate gas when abnormal charging increases the battery voltage and generates heat. Such production of carbonate gas generally causes the can to swell excessively. The safety vent is actuated when the can swells severely and prevents the explosion and/or firing of the battery.
The swelling of the can may be avoided by reducing the amount of lithium carbonate added. However, cobalt oxide (CoO2) then remains in the positive electrode active materials and corrodes the positive electrode, which dissolves into the electrolyte during charging. This causes cobalt precipitation to the negative electrode, which increases the possibility of an internal short circuit. As such, the excessive addition of Li2CO3 is inevitable.
As mentioned above, the safety vent is not actuated until the battery pressure reaches a predetermined level. However, temperature, as well as battery pressure, generally increase during overcharging. Therefore, safety can be additionally improved if the safety vent is actuated in response not only to pressure, but also to temperature.
However, the conventional safety vent formed on the cap assembly or the can with a reduced thickness, as mentioned above, is actuated in response only to the battery pressure, and not to the battery temperature. Thus , there is a need for a safety vent that may be actuated in response to battery pressure as well as battery temperature.